The present invention relates to an antenna arrangement comprising a number of radiating elements of which some radiate at a first frequency or in a first frequency band and some radiate at a second frequency or in a second frequency band so that one and the same antenna arrangement can be used for different frequencies or frequency bands.
The invention also relates to a base station antenna arrangement that can be used for a first and a second frequency band so that one and the same base station antenna arrangement can be used for different mobile communication systems operating in different frequency bands.
The field of mobile telecommunications is rapidly growing in a large number of countries and new markets and more countries are constantly introducing cellular communication systems. Furthermore new services and applications are continuously introduced on the, in every aspect, strongly expanding mobile telecommunication market. It is well known that a number systems operating in approximately the 900 MHz frequency band, for example NMT 900, (D)-AMPS, TACS, GSM and PDC, have been very successful. This has among other things had as a consequence that systems operating in other frequency bands are needed. Therefore new systems have been designed for the frequency bands around 1800 MHz and 1900 MHz. Examples thereon are DCS 1800 and PCS 1900. There are of course also a number of other systems in the 900 MHz band (and there around) as well as in the 1800 or 1900 MHz and similar which have not been explicitly mentioned herein. Bearing the recent development in mind, it is also clear that still further systems will be developed.
However, for the operation of cellular mobile telecommunication systems a large number of base station antenna installations have been necessary. Base station antenna arrangements have to be provided all over the area that is to be covered by the cellular communication system and how they are arranged among other things depends on the quality that is required and the geographical coverage, the distribution of mobile units etc. Since radio propagation depends very much on terrain and irregularities in the landscape and the cities the base station antenna arrangements have to be arranged more or less closely.
However, the installation of base station antennas has caused protests among others from an esthetical point of view both on the countryside and in the cities. Already the installation of masts with antennas for e.g. the 900 MHz frequency band has given rise to a lot of discussions and protests. The installation of additional base station antenna arrangements for another frequency band would cause even more opposition and it would indeed in some cases give rise to inconveniences, not only from the esthetical point of view. Still further the construction of antenna arrangements is expensive.
The introduction of new base station antenna arrangements would be considerably facilitated if the infrastructure that already is in place for for example the 900 MHz frequency band could be used. Since both systems operating in the lower as well as in the higher frequency band furthermore will be used in parallel, it would be very attractive if the antennas for the different frequency bands could coexist on the same masts and particularly use (share) the same antenna aperture. Today various examples of microstrip antenna elements which are capable of operating in two distinct frequency bands are known. One way of achieving this consists in stacking patches on top of each other. This works satisfactorily if the different frequency bands are spaced closely e.g. up to a ratio of about 1.5:1. However, this concept does not work when the frequency bands are less closely spaced. An example thereon is a stacked dual frequency patch element comprising a ground plane, on which e.g. a circular or a rectangular low frequency patch is arranged and on top of which a high frequency patch of a similar shape is arranged. In still another known structure, as for example disclosed in "Dual band circularly polarised microstrip array element" by A. Abdel Aziz et al, Proc. Journe'es Internationales de Nice sur les Antennes (JINA 90), pp 321-324, Nov. 1990, School of El. Engineering and Science Royal Military College of Science, Shrivenham, England, a large low frequency patch element is provided in which a number of windows (four windows) are provided. In these windows smaller patch elements are arranged. The windows do not significantly perturb the characteristics of the larger patch element. Through this arrangement it is possible to use one and the same antenna arrangement for two different frequency bands, which however are separated by a factor four. This is a frequency band separation which is much too high to be used for the, today, relevant mobile communication systems operating at about 900 MHz and 1800 (1900-1950) MHz.
Still another known technique uses the frequency selective nature of periodic structures. It has been shown that when a low frequency patch element is printed as a mesh conductor or as a perforated screen, it can be superimposed on top of another array antenna operating at a higher frequency, c.f. e.g. "Superimposed dichroic microstrip antenna arrays" by J. R. James et al, IEE Proceedings, Vol. 135, Pt. H, No.5, October 1988. This works satisfactorily for dual band operations where the bands are still more separated than in the preceding case, thus having ratios exceeding 6:1. Furthermore U.S. Pat. No. 5,001,493 shows a multiband gridded focal plane array antenna providing simultaneous beams of multiple frequencies. A metallization pattern provides a first set of conductive edges of a first length and a second set of conductive edges having a second length. The first and second sets of conductive edges are separately fed to provide first and second simultaneously output beams at the first and second operating frequencies. However, also here it is not possible to have the frequency band separation that is about two thus being useful for the mobile communication systems referred to above. U.S. Pat. No. 5,001,493 shows second radiating elements radiating at an intermediate second frequency being 2.3 times a first frequency and the third radiating elements radiating at a high frequency being about 1.1 times the second frequency. Thus the antenna arrangement as disclosed in said document is not applicable to the mobile communication systems referred to above or in general where the frequency band separation is about a factor two.
In array antennas, the element periodicity is between 0.5 and 1 free space wavelengths. The smaller spacing is used in scanned array antennas. The number of radiating elements in the 1800/1900 MHz band will be twice as many as in the 900 MHz band if the same area is utilised. This means that the high frequency antenna will have between 3 and 6 dB higher gain than the low frequency antenna. This offsets partly the increased path losses at higher frequencies making the coverage areas similar for the two bands.
Diversity antenna configurations are used today to reduce fade effects. Receive diversity at the base station is achieved with two antennas separated a couple of meters. Today, mainly vertically polarised transmit and receive antennas are employed. Polarisation diversity is another way to reduce fade effects.